Electromagnetic-wave-absorbing particles, electromagnetic-wave-absorbing particle dispersion liquids, and manufacturing methods of electromagnetic-wave-absorbing particles

ABSTRACT

Electromagnetic wave absorbing particles are provided that include hexagonal tungsten bronze having oxygen deficiency, wherein the tungsten bronze is expressed by a general formula: M x WO 3-y  (where one or more elements M include at least one or more species selected from among K, Rb, and Cs, 0.15≤x≤0.33, and 0&lt;y≤0.46), and wherein oxygen vacancy concentration N V  in the electromagnetic wave absorbing particles is greater than or equal to 4.3×10 14  cm −3  and less than or equal to 8.0×10 21  cm −3 .

TECHNICAL FIELD

The present invention relates to electromagnetic wave absorbingparticles, electromagnetic wave absorbing particle dispersion liquids,and manufacturing methods of electromagnetic wave absorbing particles.

BACKGROUND ART

According to the fifth edition of the Physical and Chemical Dictionary,light is defined as “Electromagnetic waves with wavelengths in the rangeof about 1 nm to 1 mm are called light”. This range of wavelengthsincludes the visible light region and the infrared region.

As a light shielding member used for windows and the like, in PatentDocument 1, a light shielding film is proposed that contains ablack-based pigment including an inorganic pigment such as carbon blackand titanium black that absorbs rays in the visible light region to thenear-infrared region; and an organic pigment such as aniline black thatabsorbs rays strongly only in the visible light region.

Also, in Patent Document 2, a half-mirror-type light shielding member isproposed on which metal such as aluminum is deposited.

In Patent Document 3, a heat shielding glass is proposed that has, on atransparent glass substrate, a composite tungsten oxide film containingat least one species of metal ions selected from among a groupconsisting of Group IIIa, Group IVa, Group Vb, Group VIb, and Group VIIbof the periodic table, which is provided as the first layer counted fromthe substrate side; a transparent dielectric film as the second layerprovided on the first layer; and a composite tungsten oxide filmcontaining at least one species of metal ions selected from among agroup consisting of Group IIIa, Group IVa, Group Vb, Group VIb, andGroup VIIb of the periodic table, as the third layer provided on thesecond layer. This heat shielding glass has a refractive index of thetransparent dielectric film as the second layer that is smaller than therefractive indices of the composite tungsten oxide films as the firstlayer and the third layer, and thereby, can be suitably used as a partthat is required to have high visible light transmittance and goodheat-ray shielding performance.

In Patent Document 4, a heat-ray shielding glass is proposed in which,on a transparent glass substrate, in substantially the same way as inPatent Document 3, a first dielectric film is provided as the firstlayer counted from the substrate side; a tungsten oxide film is providedon the first layer as the second layer; and a second dielectric film isprovided as the third layer on the second layer.

In Patent Document 5, a heat-ray shielding glass is proposed in which,on a transparent substrate, in substantially the same way as in PatentDocument 3, a composite tungsten oxide film containing similar metallicelements is provided as the first layer counted from the substrate side;and a transparent dielectric film is provided as the second layer on thefirst layer.

In Patent Document 6, a solar controlling glass sheet is proposed thatis formed to be covered by a metal oxide film that is formed by CVD orspraying, applied with pyrolysis at around 250° C. to have a solarshielding characteristic, where the metal oxide film is selected fromamong one or more species of tungsten trioxide (WO₃), molybdenumtrioxide (MoO₃), niobium pentoxide (Nb₂O₅), tantalum pentoxide (Ta₂O₅),vanadium pentoxide (V₂O₅), and vanadium dioxide (VO₂) that contain anadditive material such as hydrogen, lithium, sodium, or potassium.

Meanwhile, the applicant has disclosed in Patent Document 7 a fineparticle dispersoid of an infrared-shielding material in which fineparticles of the infrared-shielding material are dispersed in a medium,wherein the fine particles of the infrared-shielding material containfine particles of tungsten oxide and/or fine particles of compositetungsten oxide; and the diameter of dispersed particles of the fineparticles of the infrared-shielding material is greater than or equal to1 nm and less than or equal to 800 nm.

RELATED-ART DOCUMENTS Patent Documents

-   [Patent Document 1] Japanese Patent Application Publication No.    2003-029314-   [Patent Document 2] Japanese Patent Application Publication No.    H9-107815-   [Patent Document 3] Japanese Patent Application Publication No.    H8-59300-   [Patent Document 4] Japanese Patent Application Publication No.    H8-12378-   [Patent Document 5] Japanese Patent Application Publication No.    H8-283044-   [Patent Document 6] Japanese Patent Application Publication No.    2000-119045-   [Patent Document 7] International Publication No. 2005/037932

Non-Patent Documents

-   [Non-Patent Document 1] A. D. Walkingshaw, N. A. Spaldin, and E.    Artacho, Density-functional study of charge doping in WO₃, Phys.    Rev. B 70 (2004) 165110-1-7-   [Non-Patent Document 2] J. Oi, A. Kishimoto, T. Kudo, and M.    Hiratani, Hexagonal tungsten trioxide obtained from    peroxo-polytungstate and reversible lithium electron-intercalation    into its framework, J. Solid State Chem. 96 (1992) 13

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

Meanwhile, in recent years, while reducing the solar transmittance of,for example, glass for automobiles, for electromagnetic waves in thenear-infrared region including a wavelength of 850 nm used for variousinfrared sensors, demand has been increasing for higher transmittance.

Patent Document 7 described above discloses fine particles of tungstenoxide and/or fine particles of composite tungsten oxide that are capableof absorbing electromagnetic waves in the infrared region. However,there is no disclosure of a guideline for selecting composite tungstenoxide having excellent transmittance of near-infrared rays havingwavelengths around 850 nm to be used for various infrared sensors whilereducing the solar transmittance.

In one aspect of the present invention, it is an object to provideelectromagnetic wave absorbing particles having an excellenttransmission characteristic for near-infrared rays having a wavelengthof 850 nm while controlling the solar transmittance.

Means to Solve the Problem

According to one aspect of the present invention, electromagnetic waveabsorbing particles are provided that include hexagonal tungsten bronzehaving oxygen deficiency, wherein the tungsten bronze is expressed by ageneral formula: M_(x)WO_(3-y) (where one or more elements M include atleast one or more species selected from among K, Rb, and Cs,0.15≤x≤0.33, and 0<y≤0.46), and wherein oxygen vacancy concentrationN_(V) in the electromagnetic wave absorbing particles is greater than orequal to 4.3×10¹⁴ cm⁻³ and less than or equal to 8.0×10²¹ cm⁻³.

Advantage of the Invention

In one aspect of the present invention, it is possible to provideelectromagnetic wave absorbing particles having an excellenttransmission characteristic for near-infrared rays having a wavelengthof 850 nm while controlling the solar transmittance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of correlation between the compositionof tungsten bronze and the lattice constants; and

FIG. 2 is an explanatory diagram of correlation between the compositionof tungsten bronze and the lattice constants in the case of the elementM being Cs.

MODE FOR CARRYING OUT THE INVENTION

[Electromagnetic Wave Absorbing Particles]

In the present embodiment, an example of a configuration ofelectromagnetic wave absorbing particles will be described.

Electromagnetic wave absorbing particles according to the presentembodiment can contain hexagonal tungsten bronze having oxygendeficiency. In addition, such tungsten bronze is expressed by a generalformula: M_(x)WO_(3-y) (where one or more elements M include at leastone or more species selected from among K, Rb, and Cs, 0.15≤x≤0.33, and0<y≤0.46) and can have an oxygen vacancy concentration N_(V) greaterthan or equal to 4.3×10¹⁴ cm⁻³ and less than or equal to 8.0×10²¹ cm⁻³.

Conventionally, hexagonal M_(x)WO₃ has been proposed as tungsten bronze.Hexagonal tungsten bronze, has a structure in which ions of theelement(s) M, such as an alkali metal, are inserted in gaps of largehexagonal tunnels formed in a basic skeleton in octahedrons of WO₆ (W:tungsten, O: oxygen) as basic units are connected at the corners.

The maximum number of atoms of the element(s) M that can enter thehexagonal tunnels is 0.33 for W, and this value has been targetedpractically.

On the other hand, the oxygen deficiency of tungsten bronze has not beenunderstood in detail. Therefore, for tungsten bronze, the chemicalformula of M_(x)WO₃ that expresses the full stoichiometric ratio ofoxygen is well established regardless of the production method. In otherwords, in a tungsten bronze structure, it has been generally understoodthat the oxygen deficiency is essentially zero or in a small amount evenif it is present.

However, in the primitive form of tungsten oxide (WO₃) that does notcontain the element(s) M, it has been reported that reduction treatmenteasily generates WO_(3-x) in which the stoichiometric ratio of oxygen isdecreased, and depending on the degree of reduction, different crystalstructures, conductivities, and optical properties are observed. InWO_(3-x) referred to as the so-called “Magneli phase”, deviation fromthe stoichiometric ratio is compensated by, not oxygen vacancies, butW-rich surface defects called a crystallographic shear plane.

One of the reasons why tungsten bronze has not been examinedquantitatively with respect to the relationship between the presence ofoxygen deficiency and its structure would be, unlike tungsten oxide, thefact that the presence of such surface defects has not been noticeablyobserved with a TEM (transmission electron microscope) or the like.

The inventors of the present invention focused on and investigatedoxygen deficiency as a method of making electromagnetic wave absorbingparticles having excellent near-infrared transmission characteristicswith respect to a wavelength of 850 nm while controlling the solartransmittance.

The inventors of the present invention produced by way of trial a seriesof hexagonal M_(x)WO₃ in a strongly reducible atmosphere; analyzed thecompositions; and studied the crystal structures and optical properties.As a result, the inventors of the present invention found that asignificantly great amount of oxygen deficiency could be introduced intothe tungsten bronze. Therefore, the tungsten bronze is denoted with thegeneral formula: M_(x)WO_(3-y).

According to the investigations conducted by the inventors of thepresent invention, in tungsten bronze, it is possible to reduce thesolar transmittance by introducing the element(s) M and oxygendeficiency.

The inventors of the present invention found that by introducing andincreasing the element(s) M, and introducing and increasing the oxygendeficiency, the amount of free electrons and localized electronsincreases. More specifically, by introducing and increasing theelement(s) M, only the supply of free electrons increases. On the otherhand, the inventors of the present invention found that by introducingand increasing the oxygen deficiency, free electrons and localizedelectrons are supplied in a ratio of approximately 2:8.

Therefore, by introducing and increasing the element(s) M, andintroducing and increasing the oxygen deficiency, both the plasmonabsorption and the polaron absorption become greater, and thereby, andthe electromagnetic wave absorption effect becomes greater. In otherwords, it is possible to control the solar transmittance.

According to the investigations conducted by the inventors of thepresent invention, the plasmon absorption and the polaron absorption oftungsten bronze appear in the near-infrared region, which is greaterthan or equal to 0.5 eV and less than or equal to 1.8 eV (greater thanor equal to 690 nm and less than or equal to 2480 nm).

The plasmon absorption appears at a slightly lower energy level than thepolaron absorption, and the polaron transition wavelength of tungstenbronze is present in the vicinity of red in the visible light region tothe near-infrared region, or a region greater than or equal to 1.0 eVand less than or equal to 1.8 eV (greater than or equal to 690 nm andless than or equal to 1240 nm). Therefore, in order to increase thenear-infrared transmittance at the wavelength of 850 nm whilecontrolling the solar transmittance, it is favorable to control thepolaron absorption.

Further, according to the investigations conducted by the inventors ofthe present invention, making the amount of oxygen deficiency smallerenables to control the polaron absorption; therefore, it is favorable tocontrol the oxygen deficiency not to become excessively great.

Thereupon, electromagnetic wave absorbing particles according to thepresent embodiment can contain hexagonal tungsten bronze having oxygendeficiency. Note that electromagnetic wave absorbing particles accordingto the present embodiment can also be constituted with such tungstenbronze. Moreover, such tungsten bronze is expressed by a generalformula: M_(x)WO_(3-y), and the oxygen vacancy concentration (oxygenvacancy density) N_(V) can be set greater than or equal to 4.3×10¹⁴cm⁻³.

A lower oxygen vacancy concentration can make the near-infraredtransmittance at the wavelength of 850 nm higher. However, N_(V) cannotbe lower than the thermal equilibrium oxygen vacancy concentration thatinevitably exists at that temperature. Here, the thermal equilibriumvacancy concentration in tungsten bronze is estimated to beapproximately 4.3×10¹⁴ cm⁻³. Free electrons from alkaline ions cause theplasmon absorption even in the case of N_(V) at the lower limit of4.3×10¹⁴ cm⁻³; therefore, it is possible to sufficiently reduce thesolar transmittance. On the other hand, if oxygen deficiency isintroduced excessively, the near-infrared transmittance at thewavelength of 850 nm may be decreased. Therefore, it is favorable thatthe oxygen vacancy concentration is less than or equal to 8.0×10²¹ cm⁻³.It is more favorable that the oxygen vacancy concentration is greaterthan or equal to 4.0×10²¹ cm⁻³ and less than or equal to 8.0×10²¹ cm⁻³.

The oxygen vacancy concentration can be calculated from the compositionof tungsten bronze contained in electromagnetic wave absorbing particlesaccording to the present embodiment and the lattice constants.

As described above, oxygen deficiency in the composition of the entiretyof tungsten bronze can be designated by y in the general formuladescribed above. However, the crystal lattice of tungsten bronze isdistorted by the Jahn-Teller effect, the volume of a unit cell alsochanges with x and y. Therefore, in order to obtain an excellenttransmission characteristic of near-infrared rays having a wavelength of850 nm while controlling the solar transmittance, it is necessary thatnot only the value of y, but also the oxygen vacancy concentration fallwithin the above ranges, respectively.

Note that in the case of further increasing the near-infraredtransmittance at the wavelength of 850 nm, a method may be adopted thatalso controls the plasmon absorption. In this case, in addition to theoxygen deficiency and the oxygen vacancy concentration described above,it is favorable that the amount of the element(s) M to be added is alsoadjusted so that the near-infrared transmittance at the wavelength of850 nm falls within the desired range.

In the above general formula that expresses tungsten bronze contained inelectromagnetic wave absorbing particles according to the presentembodiment, the element(s) M can contain at least one or more speciesselected from among K, Rb, and Cs. This is because the element(s) Mcontaining one or more species selected from among K, Rb, and Cs makesit easier to form a hexagonal tungsten bronze structure.

It is also possible that the element(s) M further includes one or morespecies selected from among Na, Tl, In, Li, Be, Mg, Ca, Sr, Ba, Al, andGa as an additive element(s).

In the above general formula, x represents the ratio of the element(s) Mto W (atomic ratio), and denotes the amount of the element(s) M to beadded, which can be set to be greater than or equal to 0.15 and lessthan or equal to 0.33. This is because in the case where all the gaps inthe hexagonal tunnels are occupied by the element(s) M, x becomes 0.33.This is also because by setting x to be greater than or equal to 0.15,the solar transmittance can be controlled sufficiently, which isfavorable.

Also, in the above general formula, y represents the amount of oxygendeficiency in 3−y representing a ratio of 0 to W (atomic ratio), whichcan be set to be greater than 0. This is because by introducing oxygendeficiency, the solar transmittance can be controlled sufficiently,which is favorable. However, if oxygen deficiency is introducedexcessively, decomposition to lower oxides such as WO₂ or metal Wbegins, and this may make it difficult to maintain the hexagonalskeleton. Also, the near-infrared transmittance at the wavelength of 850nm may be decreased excessively. Therefore, in order to securelymaintain the hexagonal crystal skeleton and to increase thetransmittance of near-infrared light having a wavelength of 850 nm, itis favorable that y indicating the oxygen deficiency is less than orequal to 0.46.

Also, the inventors of the present invention have conducted furtherinvestigations, and found that the added amount of the element(s) M andthe amount of oxygen deficiency correlate with the lattice constants ofthe tungsten bronze crystal.

In tungsten bronze, d-electrons are imperfectly filled and crystalanisotropy appears; therefore, when W-5d forms an antibonding hybridorbit with O-2p, in order to compensate for an increase in the crystalenergy, so-called “Jahn-Teller distortion” is generated, in which theatomic positions of the element(s) M and O change by minute amounts.

Due to the Jahn-Teller effect, in tungsten trioxide (WO₃) as theprimitive form of tungsten bronze, the crystal normally expected to becubic is distorted into a monoclinic crystal. Walkingshaw et al. haveshown by DFT (density functional theory) calculation (Non-PatentDocument 1) that when electrons are supplied to this W-5d/O-2pantibonding hybrid orbit, the Jahn-Teller distortion is alleviated, andthe crystal structure and the lattice constants change systematically.Their calculation results reproduce change in the crystal structure ofNa_(x)WO₃ that has been experimentally observed, namely, when xincreases from 0 to 1, the crystal structure changes from a monocline toan orthorhombus, a tetragon, and a cubic crystal.

The inventors of the present invention thought that this phenomenonwould also be observed in hexagonal tungsten bronze. In the case wherethe element(s) M contains one or more species selected from among K, Rb,and Cs in tungsten bronze expressed by the general formula M_(x)WO_(3-y)having a hexagonal structure, the ion radius of the element(s) M islarge; therefore, the WO₆ octahedron is arranged to be hexagonallysymmetrical, large hexagonal tunnels are formed therein, and the ions ofthe element(s) M are stored in gaps of the hexagonal tunnels. Further,at this time, the Jahn-Teller distortion should naturally be retained inthe skeleton formed by the WO₆ octahedrons that constitute the tungstenbronze.

Thereupon, structural changes in tungsten bronze having hexagonalcrystals expressed by the general formula M_(x)WO_(3-y) were preciselyanalyzed with a Rietveld method using XRD patterns when the added amountx of the element(s) M and the amount y of oxygen deficiency werechanged. As a result, the inventors of the present invention found thatvarious parameters characterizing the WO₆ octahedron, such as thelattice constants and the W-O distance, direct toward those of a regularoctahedron while the added amount of the element(s) M and the amount ofoxygen deficiency increases, namely, found that the Jahn-Tellerdistortion is alleviated linearly.

Here, an example of a distribution of lattice constants is illustratedin FIG. 1, in which the added amount x of the element(s) M and theamount y of oxygen deficiency are changed in a hexagonal tungsten bronzeexpressed by the formula M_(x)WO_(3-y).

In FIG. 1, a distribution of lattice constants (changes) when x and y inthe above general formula are changed is illustrated in a coordinatespace in which the horizontal axis represents the lattice constant a(Å),the vertical axis represents the lattice constant c(Å), to represent acoordinate position by (the lattice constant a, the lattice constant c)in the case of the element M being one of Cs, Rb, and K. In FIG. 1, adashed line 10 corresponds to an approximate line that is drawn inaccordance with the distribution of the lattice constants a and c when xand y are changed in the case of the element M being Cs, namely, in thecase of a hexagonal cesium tungsten bronze expressed by Cs_(x)WO_(3-y).

Dashed lines 11 and 12 correspond to approximate lines that are drawn inaccordance with the distributions of similar lattice constants and c inthe cases of the elements M being Rb and K, respectively.

From FIG. 1, it can be seen that the lattice constants change on thestraight lines corresponding to the species of the elements M as thenumerical values of x and y change. Note that according toinvestigations conducted by the inventors of the present invention, thesame behavior was observed regardless of the manufacturing method and/orthe raw materials.

Here, in FIG. 2, for Cs_(x)WO_(3-y), an example of a distribution of thelattice constants when the added amount x of the element(s) M and theamount y of oxygen vacancy is changed is illustrated. In FIG. 2, resultsare also illustrated in a coordinate space in which the horizontal axisrepresents the lattice constant a(Å), the vertical axis represents thelattice constant c(Å), to represent a coordinate position by (thelattice constant a, the lattice constant c) as in the case of FIG. 1.

Suppliers of electrons to the W-5d orbit of the crystal ofCs_(x)WO_(3-y) to alleviate the Jahn-Teller distortion are Cs⁺ ions asions of the element M and the oxygen deficiency V_(o), and these amountshave relevance to x and y in the above formula.

In FIG. 2, when the added amount x of Cs as the element M is setconstant at 0.32, and the amount y of oxygen deficiency is changed from0 to 0.46, the distribution of lattice constants is designated withpoints 211 to 216. In other words, the points 211 to 216 correspond tothe composition that is expressed by Cs_(0.32)WO_(3-y). Specifically,3-y in the formula corresponds to 3.00 at the point 211, 2.90 at thepoint 212, 2.80 at the point 213, 2.70 at the point 214, 2.60 at thepoint 215, and 2.54 at the point 216.

Also, in FIG. 2, when the reduction conditions are set constant as at550° C. for 1 hour in a 1% H₂—N₂ air stream, and the added amount of Csas the element M was changed from 0.19 to 0.32, the distribution oflattice constants is designated with points 221 to 228. In other words,the points 221 to 228 correspond to the composition that is expressed byCS_(x)WO_(2.63-2.76). When the amount of Cs decreases, even under thesame amount of oxygen deficiency, the molar ratio to the entiretychanges; therefore, the O/W changes from 2.63 to 2.76. Then, x in theformula corresponds to 0.19 at the point 221, 0.20 at the point 222,0.22 at the point 223, 0.24 at the point 224, 0.26 at the point 225,0.28 at the point 226, 0.30 at the point 227, and 0.32 at the point 228.

An approximation line was drawn for the distribution of latticeconstants described above, and it was confirmed that all points werearranged almost linearly as illustrated in FIG. 2.

In addition, it was confirmed that the lattice constants weredistributed along the following formula (A).

c(Å)=−3.436a(Å)+33.062  (A)

Also, the constants move toward the lower right side of the approximateline when the Jahn-Teller distortion is large, and move toward the upperleft side when the distortion is small.

When the reduction conditions such as oxygen reduction time are setconstant and the added amount of Cs as the added amount of the element Mis increased, the constants change significantly from the point 221 tothe point 228 in FIG. 2, namely, within a region X in FIG. 2.

On the other hand, when the added amount of Cs as the added amount ofthe element M is set constant at 0.32 and the amount y of oxygendeficiency is increased, the constants change from the point 211 to thepoint 216 in FIG. 2, namely, in a small range within a region Y in FIG.2.

When cesium tungsten bronze is decreased so as to increase the amount ofoxygen deficiency to be greater than in the case of the point 216,although decomposition to WO₂ as a lower oxide and metal W begins, theamount y of oxygen deficiency in cesium tungsten bronze does not gobeyond the case of the point 216.

Comparing the lengths of the region X and the region Y, it is obviousthat the change in the lattice constants when the amount y of oxygendeficiency changes from the point 211 to the point 216 by 0.46, issmaller than the change in the lattice constants when the added amountof Cs as the added amount x of the element M changes from the point 221to the point 228 by 0.13. Specifically, the length of the region Y is1/20.6 times the length of the region X.

This is because the amount of free electrons released by theintroduction of the oxygen deficiency V_(o) is smaller than the amountof free electrons released by the addition of the element M.

Jahn-Teller distortion depends only on the amount of electrons;therefore, the chart illustrated in FIG. 2 can be used for any values ofx and y as long as the element M remains the same as Cs.

For example, when the amount of oxygen deficiency is set constant at themaximum (y=0.46), the change in the lattice constants when the addedamount x of Cs as the added amount of the element M changes between 0.19and 0.32 corresponds to a change from the point 222 to the point 216 inthe figure. This is because it is possible to derive a range of thelattice constants without changing the width of region X as the changewidth when the added amount of Cs as the added amount of the element Mis changed, by sliding the range so as to start from the point 216 wherethe content of oxygen deficiency is maximum.

In the case where the added amount x of Cs as the added amount of theelement M is set constant at 0.26 and the amount y of oxygen deficiencyis changed from 0 to 0.46, it is possible to derive a range of latticeconstants without changing the width of the region Y, by sliding theregion Y so that a point of the region Y at which the amount of oxygen3-y is 2.63 corresponds to the point 225 at which the added amount x ofCs is 0.26. Note that at the point 225, the amount of oxygen 3-y is2.63; therefore, the region Y is slid so that the point at which theamount of oxygen 3-y is 2.63 corresponds to the point 225 as describedabove.

As described above, the lattice constants of tungsten bronze correspondto the composition, and for example, in the case where the added amountof the element M is known, by measuring the lattice constants, it ispossible to determine the amount y of oxygen deficiency and the like.This method is useful because tungsten bronze is very easily oxidizedand the oxidation state changes subtly depending on the raw materialsand the manufacturing conditions.

In the case of the element M being Cs, it is generally favorable thatthe lattice constant c falls within 7.576≤c≤7.610. In particular, thestrongest near-infrared absorption effect and particularly highcontrollability of the solar transmittance are obtained with constantsin a region close to the point 216 where the added amount of Cs as theelement M is large and the amount of oxygen deficiency is large. Inother words, a region where the lattice constant c falls within7.594≤c≤7.610 is more favorable.

However, as described above, if the amount of oxygen deficiency becomestoo large, the transmittance of electromagnetic waves at the wavelengthof 850 nm may be decreased. Therefore, in order to increase thenear-infrared transmittance at the wavelength of 850 nm in particularwhile controlling the solar transmittance, it is more favorable toperform control so that the added amount of Cs as the element M and theoxygen deficiency do not become excessively large. Therefore, it isfurther favorable that in the case of the element M being Cs, thelattice constant c falls within 7.594≤c≤7.600.

Here, although the case of the element M being Cs has been described asan example, as illustrated in FIG. 1, even in the case of the element Mbeing Rb or K, the lattice constants change linearly by changing thecomposition similarly.

Specifically, in the case of the element M being Rb, the constantschange along a line of c=−3.344a+32.265 designated by the dashed line11. Also, in order to increase the transmittance of near-infrared rayshaving a wavelength of 850 nm in particular while controlling the solartransmittance, it is favorable to control the oxygen deficiency so asnot to be excessively large; therefore, in the case of the element Mbeing Rb, it is favorable that the lattice constant c falls within7.517≤c≤7.580, and it is more favorable to fall within 7.517≤c≤7.560.

Also, in the case of the element M being K, the constants change along aline of c=−2.9391a+29.227 designated by the dashed line 12. Also, inorder to increase the transmittance of near-infrared rays having awavelength of 850 nm in particular while controlling the solartransmittance, it is favorable to control the oxygen deficiency so asnot to be excessively large; therefore, it is favorable that the latticeconstant c falls within 7.504≤c≤7.564, and it is more favorable to fallwithin 7.504≤c≤7.544.

Also, the element(s) M is not limited to a single species, and mayinclude multiple elements. For example, part of Cs in cesium tungstenbronze in which the element M is Cs can be substituted with Rb. In thiscase, Rb substitutes the Cs sites in the cesium tungsten bronze tobecome an all proportional solid solution.

An approximate line of the distribution of lattice constants ofcesium-rubidium tungsten bronze in which part of Cs is substituted withRb, depending on the amount of substitution with Rb, for example, inFIG. 1, corresponds to a line obtained by translating the dashed line 10as an approximate line of the distribution of lattice constants ofcesium tungsten bronze in the direction toward the dashed line 11 as anapproximate line of the distribution of lattice constants of rubidiumtungsten bronze.

Similarly, in the case where part of Rb in rubidium tungsten bronze issubstituted with K, an approximate line of the distribution of latticeconstants of rubidium-potassium tungsten bronze, depending on the amountof substitution with K, for example, in FIG. 1, corresponds to a lineobtained by translating the dashed line 11 as an approximate line of thedistribution of lattice constants of rubidium tungsten bronze in thedirection toward the dashed line 12 as an approximate line of thedistribution of lattice constants of potassium tungsten bronze.

Tungsten bronze contained in electromagnetic wave absorbing particlesaccording to the present embodiment contains at least one or morespecies selected from among K, Rb, and Cs as the element(s) M asdescribed above. A tungsten bronze structure also appears when addedwith not only one or more species selected from among K, Rb, and Cs, butalso the other alkali metals, Li and Na; In and Tl as the 13th group ofelements; Be, Mg, Sr, Ba, and Ca as the alkaline earth metals; Ti, V,Zr, and Nb as the transition metals; Cu, Ag, Au, and Pt as the preciousmetals; and the like. Therefore, the element(s) M can further includeone or more species selected from among Na, Tl, In, Li, Be, Mg, Ca, Sr,Ba, Al, and Ga as an additive element(s). According to investigationsconducted by the inventors of the present invention, in the case ofadding these additive elements, in order to increase the transmittanceof near-infrared rays having a wavelength of 850 nm in particular whilecontrolling the solar transmittance, it is favorable that the latticeconstants are distributed between the dashed line 10 and the dashed line12 in FIG. 1.

In particular, it is more favorable that the lattice constants arepositioned in the quadrangular region ABCD connecting the pointsA(7.406, 7.614), B(7.372, 7.564), C(7.393, 7.504), and D(7.423, 7.554).

However, lattice constants calculated from XRD patterns are influencedby various factors not only by the history, shape, size, anddistribution of particle sizes of the target material itself, but alsothe device used for XRD measurement, the width of an x-ray slit, spreadof diffraction lines, and the type of reference samples, and the like.The inventors of the present invention obtain data using a preciselycalibrated device, and in the case of cesium tungsten bronze andrubidium tungsten bronze, based on a space group P6₃/mcm, a Rietveldanalysis is carried out to calculate the lattice constants. Also, in thecase of potassium tungsten bronze, a Rietveld analysis is carried outbased on a space group P6₃22 to calculate the lattice constants.

However, industrially, it is favorable to design the lattice constantstaking into account variation. Therefore, for each point in the aboveregion, it is favorable to consider a width of ±0.006 Å for the latticeconstant a, and a width of ±0.01 Å for the lattice constant c.

Also, according to investigations conducted by the inventors of thepresent invention, the lattice constants may change in the case whereelectromagnetic wave absorbing particles become finer particles. Forexample, in the case of applying a crushing process to a bulk tomechanically produce nanoparticles, the oxygen deficiency is decreaseddue to the influence of strong pressure and shear deformation. Also, inthe case of tungsten bronze, elimination regions of the element M appearon the particle surfaces. These changes decrease x and y in the generalformula of tungsten bronze described above, and thereby, the Jahn-Tellerdistortion becomes greater; therefore, as for the lattice constants, itwas expected that the lattice constant in the a-axis direction wouldincrease, whereas the lattice constant in the c-axis direction woulddecrease. However, what the experiment showed was the opposite.Specifically, as for the lattice constants, while the particles becomefiner, it was confirmed that the lattice constant in the a-axisdirection was decreased along the line illustrated in FIG. 1, and thelattice constant in the c-axis was displaced in the increasingdirection.

As a result of experiments conducted by the inventors of the presentinvention with respect to nanoparticles ranging from greater than orequal to 10 nm and less than or equal to 40 nm, the mean displacementsof the lattice constants compared with those before becoming finerparticles were Δa=−0.0024 Å and Δc=+0.0084 Å. This indicates that theJahn-Teller distortion is significantly alleviated by the crystal sizeeffect when the particle diameter becomes nanosized from a bulk.Therefore, in addition to the industrial variation described above, itis favorable to take into account changes in the lattice constants inthe case of finer particles, by considering, for each point in the aboveregion, a width of ±0.0084 Å for the lattice constant a, and a width of±0.0184 Å for the lattice constant c.

As described above, it is favorable that a_(M-HTB) (Å) and c_(M-HTB)(Å)as the lattice constants a and c for hexagonal tungsten bronze containedin electromagnetic wave absorbing particles according to the presentembodiment, satisfies the following rules.

In the coordinate space wherein the horizontal axis represents thelattice constant a(Å), the vertical axis represents the lattice constantc(Å), to represent a coordinate position by (the lattice constant a, thelattice constant c), it is more favorable that the lattice constantshave the following relationships of formulas (1) and (2) with a point(a_(M), c_(M)) positioned in the quadrangular region ABCD connecting thepoints A(7.406, 7.614), B(7.372, 7.564), C(7.393, 7.504), and D(7.423,7.554).

a _(M-HTB) =a _(m)±0.0084  (1)

C _(M-HTB) =c _(M)±0.0184  (2)

The lattice constants of tungsten bronze contained in electromagneticshielding particles according to the present embodiment falling withinpredetermined ranges in this way mean that the element(s) M and theoxygen deficiency fall within the predetermined ranges at the level ofatomic arrangement. Therefore, it is possible to obtain electromagneticwave absorbing particles having excellent near-infrared transmissioncharacteristics with respect to the wavelength of 850 nm whilecontrolling the solar transmittance, which is favorable.

In particular, in the case of cesium tungsten bronze in which theelement M is Cs, in the coordinate space wherein the horizontal axisrepresents the lattice constant a(Å), the vertical axis represents thelattice constant c(Å), to represent a coordinate position by (thelattice constant a, the lattice constant c), it is favorable fora_(M-HTB)(Å) and c_(M-HTB)(Å) as the lattice constants a and c for thehexagonal tungsten bronze, to have the following relationships offormulas (3) and (4) with a point (a_(Cs), c_(Cs)) positioned on astraight line of c_(Cs)=−3.436a_(Cs)+33.062.

a _(M-HTB) =a _(Cs)±0.0084  (3)

C _(M-HTB) =c _(Cs)±0.0184  (4)

Note that in order to increase the transmittance of near-infrared rayshaving a wavelength of 850 nm in particular while controlling the solartransmittance, it is more favorable for c_(Cs) to satisfy7.576≤c_(Cs)≤7.610.

Also, in the case of rubidium tungsten bronze in which the element M isRb, in the coordinate space wherein the horizontal axis represents thelattice constant a(Å), the vertical axis represents the lattice constantc(Å), to represent a coordinate position by (the lattice constant a, thelattice constant c), it is favorable for a_(M-HTB)(Å) and c_(M-HTB)(Å)as the lattice constants a and c for the hexagonal tungsten bronze, tohave the following relationships of formulas (5) and (6) with a point(a_(Rb), c_(Rb)) positioned on a straight line ofc_(RB)=−3.344a_(Rb)+32.265.

a _(M-HTB) =a _(Rb)±0.0084  (5)

c _(M-HTB) =c _(R) b±0.0184  (6)

Note that in order to increase the transmittance of near-infrared rayshaving a wavelength of 850 nm in particular while controlling the solartransmittance, it is more favorable for c_(Rb) to satisfy7.517≤c_(Rb)≤7.580.

Also, in the case of potassium tungsten bronze in which the element M isK, in the coordinate space wherein the horizontal axis represents thelattice constant a(Å), the vertical axis represents the lattice constantc(Å), to represent a coordinate position by (the lattice constant a, thelattice constant c), it is favorable for a_(M-HTB)(Å) and c_(M-HTB)(Å)as the lattice constants a and c for the hexagonal tungsten bronze, tohave the following relationships of formulas (7) and (8) with a point(a_(K), c_(K)) positioned on a straight line ofc_(K)−2.9391a_(K)+29.227.

a _(M-HTB) =a _(K)±0.0084  (7)

c _(M-HTB) =c _(K)±0.0184  (8)

Note that in order to increase the transmittance of near-infrared rayshaving a wavelength of 850 nm in particular while controlling the solartransmittance, it is more favorable for c_(K) to satisfy7.504≤c_(K)≤7.564.

The interatomic distance also reflects alleviation of the Jahn-Tellerdistortion caused by the addition of the element(s) M and introductionof oxygen deficiency. As described above, the Jahn-Teller distortion isretained in the skeleton formed by WO₆ octahedrons in tungsten bronzehaving a hexagonal structure, and the WO₆ octahedrons are not isotropicoctahedrons. In other words, the distances from a W atom at the centerto the surrounding O atoms are different from each other.

For example, Oi et al. synthesized a hexagonal tungsten oxide (h-WO₃)without the dope element M and oxygen deficiency, which corresponds tox=0 and y=0, and carried out a Rietveld analysis assuming the spacegroup P6₃/mcm to obtain the lattice constants, atomic positions, andinteratomic distances in the crystal phase (Non-Patent Document 2).According to this report, among the W-O distances in a WO₆ octahedron,there are two distances from [an O atom present in the c-axis directionwhen viewed from the W atom] to [the W atom at the center], which are1.80 Å and 2.13 Å, respectively, and the ratio is 1.18. In other words,in the case where there is no supply of electrons by the dope element Mand oxygen deficiency, the ratio of the W-O distances becomes greaterdue to the Jahn-Teller distortion.

As described above, the inventors of the present invention analyzedstructural changes in tungsten bronze having hexagonal crystalsexpressed by the general formula M_(x)WO_(3-y), when the added amount xof the element(s) M and the amount y of oxygen deficiency were changed.Then, from a result of a precise analysis using the Rietveld method,coordinates of atoms were obtained to calculate the W-O distances in aWO₆ octahedron contained in a crystal of the tungsten bronze havinghexagonal crystals. As a result, the inventors found that while theadded amount of the element(s) M and the amount of oxygen deficiencywere increased, the Jahn-Teller distortion was linearly alleviated, andthe ratio of the W-O distances described above became smaller.

Further, the inventors found that in the WO₆ octahedron present in acrystal of hexagonal tungsten bronze, by having the ratio of the maximumvalue to the minimum value among the distances from [an O atom presentin the c-axis direction when viewed from the W atom] to [the W atom atthe center] (the maximum value/the minimum value) set within apredetermined range, the transmittance of near-infrared rays having awavelength of 850 nm can be increased in particular while controllingthe solar transmittance.

Specifically, a case of the ratio greater than or equal to 1.00 and lessthan or equal to 1.10 is favorable because it is possible toappropriately control the added amount of element M and the amount ofoxygen deficiency to increase the transmittance of near-infrared rayshaving a wavelength of 850 nm in particular while controlling the solartransmittance.

Although the mean particle diameter of electromagnetic wave absorbingparticles according to the present embodiment is not limited inparticular, it is favorable to be greater than or equal to 0.1 nm andless than or equal to 100 nm.

This is because by making the mean particle diameter of electromagneticwave absorbing particles less than or equal to 100 nm, the near-infraredabsorption characteristics can be particularly increased, namely, thesolar transmittance can be particularly controlled. This is also becauseby making the mean particle diameter of electromagnetic wave absorbingparticles greater than or equal to 0.1 nm, it becomes industriallyeasier to manufacture the particles.

Note that, for example, as in the case of windshields of automobiles, inthe case of an application in which transparency in the visible lightregion is important, it is favorable to further consider decrease ofscattering caused by electromagnetic wave absorbing particles. In thecase where decrease of scattering is important, it is particularlyfavorable that the mean particle diameter of electromagnetic waveabsorbing particles is less than or equal to 30 nm.

The mean particle diameter means a particle diameter at a cumulativevalue of 50% in a particle size distribution, and means the same also inthe other parts of the present description. As a method of measuring aparticle size distribution to calculate the mean particle diameter,direct measurement of the particle diameter for each particle using, forexample, a transmission electron microscope can be used.

Also, surface treatment may be applied to electromagnetic wave absorbingparticles according to the present embodiment for purposes includingsurface protection, durability improvement, oxidation protection, waterresistance improvement, and the like. Although the specific contents ofsurface treatment are not limited in particular, for example, thesurface of an electromagnetic wave absorbing particle according to thepresent embodiment can be modified with a compound containing one ormore species of elements selected from among Si, Ti, Zr, and Al. At thistime, as the compound containing one or more species of elementsselected from among Si, Ti, Zr, and Al, one or more species selectedfrom among oxide, nitride, carbide, and the like may be listed.

[Manufacturing Method of Electromagnetic Wave Absorbing Particles]

An example of a configuration of a manufacturing method ofelectromagnetic wave absorbing particles according to the presentembodiment will be described.

Note that electromagnetic wave absorbing particles described above canbe manufactured by the manufacturing method of electromagnetic waveabsorbing particles according to the present embodiment; therefore, thedescription of some of the matters already described will be omitted.

The manufacturing method of electromagnetic wave absorbing particlesaccording to the present embodiment is not limited in particular, and avariety of methods can be used as long as a predetermined compositionand a predetermined oxygen vacancy concentration can be obtained. As themanufacturing method of electromagnetic wave absorbing particlesaccording to the present embodiment, a synthetic method in a reducinggas stream; a method of solid-phase reduction reaction; a method ofoxidizing tungsten bronze obtained by a solid-phase reaction, liquidphase method, or gas phase method using an oxygen-containing gas; amethod of applying reducing treatment using reducing gas flow; a methodof reducing WO₃ in a molten alkali halide; or the like may be listed.

(1) Synthetic Method in a Reducing Gas Stream

As the manufacturing method of electromagnetic wave absorbing particlesaccording to the present embodiment, for example, a synthetic method ina reducing gas stream can be used.In this case, the manufacturing method of electromagnetic wave absorbingparticles according to the present embodiment may include a heating stepin which a mixture of raw materials containing the element(s) M (wherethe element(s) M includes at least one or more species selected fromamong K, Rb, and Cs) and W, where the atomic ratio M/W between theelement(s) M and W is x (0.15≤x≤0.33) are heated at a solid-phasereaction temperature higher than or equal to 400° C. and lower than orequal to 650° C. in the air stream of a reducing gas to cause asolid-phase reaction.

Also, in this case, it is favorable that the manufacturing method ofelectromagnetic wave absorbing particles according to the presentembodiment further includes a homogenization step of performing heattreatment at a temperature higher than the solid-phase reactiontemperature, e.g., higher than or equal to 700° C. and lower than orequal to 900° C.

As the raw materials, for example, an aqueous solution of M₂CO₃ (anaqueous solution containing carbonate of the element M) and H₂WO₄(tungstic acid) may be used. Note that in the case where the multipleelements of M are contained, multiple corresponding aqueous solutions ofM₂CO₃ can be used.

The raw material mixture to which the heating step is to be applied isobtained by mixing and kneading the aqueous solution of M₂CO₃ and H₂WO₄at a ratio x (0.15≤x≤0.33) of M/W as the atomic ratio. Also, it isfavorable to dry the raw material mixture in the atmosphere before theheating step.

In the heating step, M_(x)WO_(3-y) as tungsten bronze can be prepared byheating at a solid-phase reaction temperature higher than or equal to400° C. and lower than or equal to 650° C. in a gas stream of a reducinggas to cause a solid-phase reaction.

In the case of x=0.33, the reaction formula is expressed by thefollowing formula:

0.165M₂CO₃+H₂WO₄+(0.165+y)H₂→0.165CO_(2↑)+(1.165+y)H₂O↑+M_(0.33)WO_(3-y)

In the homogenization step, the heat treatment may be carried out at atemperature that is higher than the solid-phase reaction temperature ofthe heating step, for example, higher than or equal to 700° C. and lowerthan or equal to 900° C. The homogenization step may be carried out inan inert gas atmosphere, for example, nitrogen, argon, or the like.

(2) Solid Phase Reduction Reaction Method

Also, electromagnetic wave absorbing particles according to the presentembodiment may be produced by a solid phase reduction reaction method.

In this case, the manufacturing method of electromagnetic wave absorbingparticles according to the present embodiment may include a heating andreduction step in which the raw material mixture is heated at atemperature higher than or equal to 750° C. and lower than or equal to950° C. under a vacuum atmosphere or inert gas atmosphere.

Note that as the raw material mixture, WO₃; one or more species selectedfrom among M₂CO₃ and M₂WO₄ (where the element(s) M includes at least oneor more species selected from among K, Rb, and Cs); and one or morespecies selected from among a simple substance of tungsten and atungsten oxide whose atomic number ratio of O/W is less than 3 may beincluded, and x as the atomic number ratio M/W of the element(s) M and Wmay be set to be 0.15≤x≤0.33.

In order to achieve the stoichiometry of the composition includingoxygen in reaction among the raw materials in a closed system, in thecase of heating in a vacuum or under an inert gas atmosphere, it isfavorable to contain a reducing component as a raw material.Specifically, as described above, as the raw materials, in addition toWO₃ and at least one species selected from among M₂CO₃ and M₂WO₄ as thereducing component as described above, it is favorable to use one ormore species selected from among a simple substance of tungsten and atungsten oxide whose atomic number ratio of O/W less than 3 (e.g., oneor more species selected from among W, NO₂, and WO_(2.72))

Then, by weighing and mixing the components of the raw materials, andperforming the heating and reduction step of heating in a vacuum orunder an inert gas atmosphere at a temperature higher than or equal to750° C. and lower than or equal to 950° C., M_(x)WO_(3-y) can besynthesized. Heating may be carried out in a vacuum-sealed quartz tube;heating may be carried out in a vacuum calcination furnace; or heatingmay be carried out in an inert gas atmosphere of nitrogen or the like.Note that in the case of using an inert gas, heating may be carried outunder an air stream of the inert gas. An ideal reaction formula in thecase of using NO₃, M₂WO₄, and a simple substance of tungsten W as theraw materials is expressed by the following formula.(x/2)M₂WO₄+(1-2x/3)WO₃+(x/6)W=M_(x)WO₃ However, although thestoichiometry of oxygen should be achieved by solid-phase reactionsamong the raw materials, normally, oxygen deficiency is introduced andM_(x)WO_(3-y) is formed. The reason for this can be attributed to M₂CO₃and M₂WO₄, each of which is one of the raw materials. M₂CO₃ and M₂WO₄tend to absorb moisture during weighing at room temperature, to becomedeliquescent. If letting the solid-phase reaction to proceed with suchraw materials, water that has been mixed in response to the moisture inthe atmosphere evaporates to act as a reducing agent; therefore,M_(x)WO_(3-y) having oxygen deficiency is generated.

M_(x)WO_(3-y) is highly reductive; therefore, ideas are required inorder to produce samples that have very low or no oxygen deficiency. Inthis case, the raw material M₂WO₄ needs to be weighed in a dryenvironment in a glove box, and handled so as not to absorb moisture.For example, in the case of heating in a vacuum calcination furnace,although it is heated and calcined at a temperature higher than or equalto 750° C. and lower than or equal to 950° C., in the case where thehold time in vacuum at high temperature is longer than several days,there may be a case where reduction of M_(x)WO_(3-y) proceeds to a fullyreduced state, namely, y becomes 0.46. Further, in the case of using acrucible of a carbon material, reduction may proceed at a partcontacting carbon.

As such, the hygroscopicity of the raw materials M₂CO₃ and M₂WO₄ and theconditions when carrying out the heat treatment in the heating andreduction step are important factors that affect the amount of oxygendeficiency in M_(x)WO_(3-y) to be obtained. Therefore, to control theamount y of oxygen deficiency in M_(x)WO_(3-y) to be a particularly lowvalue, it is favorable to pay sufficient attention to the raw materials,humidity in the atmosphere, hold time in vacuum, material of thecrucible, and the like.

(3) Method of Oxidizing or Reducing Tungsten Bronze

As the manufacturing method of electromagnetic wave absorbing particlesaccording to the present embodiment, a method of oxidizing tungstenbronze using an oxidizing gas or a method of reducing tungsten bronzeusing a reducing gas may also be listed.

In this case, the manufacturing method of electromagnetic wave absorbingparticles according to the present embodiment may include an annealingstep in which tungsten bronze including oxygen deficiency expressed bythe general formula: M_(x)WO_(3-y) (where one or more elements M includeat least one or more species selected from among K, Rb, and Cs,0.15≤x≤0.33, and 0<y≤0.46) is placed in a crucible made of anon-reducing material, and is annealed at a temperature higher than orequal to 300° C. and lower than or equal to 650° C. in an oxidizing gasatmosphere.

Also, the manufacturing method of electromagnetic wave absorbingparticles according to the present embodiment may include an annealingstep in which tungsten bronze including oxygen deficiency expressed bythe general formula: M_(x)WO_(3-y) (where one or more elements M includeat least one or more species selected from among K, Rb, and Cs,0.15≤x≤0.33, and 0<y≤0.46) is annealed at a temperature higher than orequal to 300° C. and lower than or equal to 650° C. in a reducing gasatmosphere.

In the method of oxidizing tungsten bronze, a tungsten bronze that has agreater amount of oxygen deficiency than the desired tungsten bronze maybe heated in an oxidizing gas atmosphere to decrease the amount ofoxygen deficiency so as to achieve the desired value.

As the oxidizing gas, an oxygen-containing gas may be used. Although theamount of oxygen in the oxygen-containing gas is not limited inparticular, it is favorable to contain oxygen by greater than or equalto 18 vol % and less than or equal to 100 vol %. As the oxidizing gas,it is favorable to use, for example, the atmosphere (air) or oxygen gas.

In the method of reducing tungsten bronze, a tungsten bronze that has asmaller amount of oxygen deficiency than the desired tungsten bronze maybe heated in a reducing gas atmosphere to increase the amount of oxygendeficiency so as to achieve the desired value. Note that in the case ofcarrying out the reduction, it is favorable to carry out under an airflow of the reducing gas.

As the reducing gas, a mixed gas that contains a reducing gas such ashydrogen and inert gas can be used.

Note that the manufacturing method of tungsten bronze to which oxidationor reduction is to be applied is not limited in particular.

For example, as the manufacturing method of tungsten bronze, a thermalplasma method may be used.

When synthesizing tungsten bronze by a thermal plasma method, a powdermixture of a tungsten compound and a compound of the element(s) M can beused as the raw materials.

As the tungsten compound, it is favorable to use one or more speciesselected from among tungstic acid (H₂WO₄), ammonium tungstate, tungstenhexachloride, and a tungsten hydrate obtained by adding water totungsten hexachloride dissolved in alcohol to be hydrolyzed, and then,evaporating the solvent.

Also, as the compound of the element(s) M, it is favorable to use one ormore species selected from among oxides, hydroxides, nitrates, sulfates,chlorides, and carbonates of the element(s) M.

An aqueous solution containing the tungsten compound described above andthe compound of the element(s) M described above are wet-mixed so thatthe ratio of the element(s) M and the element W becomes M_(x)W_(y)O_(z)(where M represents the element(s) M, W represents tungsten, Orepresents oxygen, 0.001≤x/y≤1.0, and 2.0≤z/y≤3.0). Then, by drying theobtained mixed solution, a powder mixture of the compound of theelement(s) M and the tungsten compound is obtained. Then, the powdermixture can be used as the raw material for the thermal plasma method.

Also, a composite tungsten oxide obtained by applying a firstcalcination step to the powder mixture under an inert gas alone or amixed gas atmosphere of inert gas and a reducing gas, can also be usedas the raw material for the thermal plasma method. Other than the above,a composite tungsten oxide obtained by applying two-step calcination tothe powder mixture, in which a first calcination step is carried outunder a mixed gas atmosphere of an inert gas and a reducing gas, and asecond calcination step is carried out under an inert gas atmosphere forthe calcined material obtained in the first calcination step, can alsobe used as the raw material for the thermal plasma method.

Although the thermal plasma used in the thermal plasma method is notlimited in particular, and it is possible to use, for example, one ormore species selected from among one of DC arc plasma, radio-frequencyplasma, microwave plasma, or low-frequency AC plasma; a superimposedplasm of these plasmas; a plasma generated by an electrical method inwhich a magnetic field is applied to a DC plasma; a plasma generated byirradiation with a high-power laser; a plasma generated by a high-powerelectron beam or ion beam; and the like.

However, whatever the thermal plasma is used, it is favorable to be athermal plasma having a high temperature part of 10000 to 15000K, inparticular, to be a plasma with which the time to generate fineparticles can be controlled.

The raw materials supplied into the thermal plasma having the hightemperature part instantaneously evaporate at the high temperature part.Then, the evaporated raw materials are condensed during the course ofreaching the tail flame part of the plasma and rapidly solidifiedoutside of the plasma flame, and thus, tungsten bronze can be generated.

Further, as the manufacturing method of tungsten bronze, a method ofsynthesizing a publicly-known compound, such as a hydrothermal synthesismethod, may be used.

The manufacturing method of electromagnetic wave absorbing particlesaccording to the present embodiment may include any step other than thesteps described above. As described above, it is favorable thatelectromagnetic wave absorbing particles according to the presentembodiment are finely processed to become finer particles.

Specific means for making finer particles is not limited in particular,and various means that can mechanically crush a substance can be used.As a mechanical crushing method, a dry crushing method using a jet millor the like may be used. Also, in the process of obtaining anelectromagnetic wave absorbing particle dispersion liquid, which will bedescribed later, mechanical crushing may be carried out in a solvent.

Also, as described above, the surfaces of electromagnetic wave absorbingparticles according to the present embodiment can be modified with acompound containing one or more species of elements selected from amongSi, Ti, Zr, and Al.

Therefore, a coating step may be provided in which, for example, analkoxide containing one or more species of metals selected from amongthe group of metals described above may be added to form coating on thesurfaces of electromagnetic wave absorbing particles according to thepresent embodiment.

[Electromagnetic Wave Absorbing Particle Dispersion Liquid andElectromagnetic Wave Absorbing Particle Dispersoid]

Next, an example of a configuration of an electromagnetic wave absorbingparticle dispersion liquid and an electromagnetic wave absorbingparticle dispersoid will be described in the present embodiment.

An electromagnetic wave absorbing particle dispersion liquid of thepresent embodiment may have a configuration that includeselectromagnetic wave absorbing particles described above, and one ormore species of liquid media selected from among water, organic solvent,oil, liquid resin, and a liquid plastic plasticizer, whereinelectromagnetic wave absorbing particles described above are dispersedin a liquid medium.

An electromagnetic wave absorbing particle dispersion liquid is a liquidin which electromagnetic wave absorbing particles described above aredispersed in a liquid medium as a solvent.

As the liquid medium, as described above, one or more species selectedfrom among water, organic solvent, oils and fats, liquid resin, andliquid plastic plasticizers can be used.

As the organic solvent, it is possible to make a selection from among avariety of solvents such as alcohol-based, ketone-based,hydrocarbon-based, glycol-based, and water-based solvents. Specifically,alcohol-based solvents such as isopropyl alcohol, methanol, ethanol,1-propanol, isopropanol, butanol, pentanol, benzyl alcohol, diacetonealcohol, and 1-methoxy-2-propanol; ketone-based solvents such asdimethyl ketone, acetone, methyl ethyl ketone, methyl propyl ketone,methy isobutyl ketone, cyclohexanone, and isophorone; ester-basedsolvents such as 3-methyl-methoxy-propionate and butyl acetate; glycolderivatives such as ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol isopropyl ether, propylene glycolmonomethyl ether, propylene glycol monoethyl ether, propylene glycolmethyl ether acetate, and propylene glycol ethyl ether acetate; amidessuch as formamide, N-methylformamide, dimethylformamide,dimethylacetamide, and N-methyl-2-pyrrolidone; aromatic hydrocarbonssuch as toluene and xylene; halogenated hydrocarbons such as ethylenechloride and chlorobenzene; and the like may be listed.

However, among these, an organic solvent having a low polarity isfavorable, and in particular, isopropyl alcohol, ethanol,1-methoxy-2-propanol, dimethyl ketone, methyl ethyl ketone, methylisobutyl ketone, toluene, propylene glycol monomethyl ether acetate,n-butyl acetate, and the like are more favorable. One or more of theseorganic solvents may be used singly or in combination.

As the oils and fats, for example, one or more species selected fromamong drying oils such as linseed oil, sunflower oil, and tung oil;semi-drying oils such as sesame oil, cottonseed oil, rapeseed oil,soybean oil, and rice bran oil; non-drying oils such as olive oil,coconut oil, palm oil, and dehydrated castor oil; fatty acid monoestersobtained by direct esterification of fatty acids of vegetable oil andmonoalcohols; ethers; and petroleum solvents such as Isopar (registeredtrademark) E, Exxsol (registered trademark) Hexane, Heptane, E, D30,D40, D60, D80, D95, D110, D130 (all manufactured by ExxonMobil) may beused.

As the liquid resin, for example, one or more species selected fromamong a liquid acrylic resin, a liquid epoxy resin, a liquid polyesterresin, and a liquid urethane resin may be used.

As the plasticizer, for example, a liquid plastic plasticizer may beused.

Also, acid or alkali may be added if necessary to adjust the pH of thedispersion liquid.

In an electromagnetic wave absorbing particle dispersion liquiddescribed above, various surfactants, coupling agents, and the like maybe added as dispersants in order to further improve dispersion stabilityof the electromagnetic wave absorbing particles and to prevent thedispersion particle size from becoming bulky due to re-cohesion.

Although the dispersants such as a surfactant, a coupling agent, and thelike can be selected in accordance with the application, it is favorableto include a group containing an amine, a hydroxyl group, a carboxylgroup, or an epoxy group, as a functional group. These functional groupsare absorbed on the surfaces of electromagnetic wave absorbing particlesto prevent the electromagnetic wave absorbing particles from cohering,and to bring an effect of uniformly dispersing the electromagnetic waveabsorbing particles in an infrared shielding film formed by using theelectromagnetic wave absorbing particles. A polymeric dispersant havingany of these functional groups in the molecules is further desirable.

As commercial dispersants, Solsperse (registered trademark) 9000, 12000,17000, 20000, 21000, 24000, 26000, 27000, 28000, 32000, 35100, 54000,250 (manufactured by Japan Lubrizol Co., Ltd.); Efka (registeredtrademark) 4008, 4009, 4010, 4015, 4046, 4047, 4060, 4080, 7462, 4020,4050, 4055, 4400, 4401, 4402, 4403, 4300, 4320, 4330, 4340, 6220, 6225,6700, 6780, 6782, 8503 (manufactured by Efka additives); Ajisper(registered trademark) PA111, PB821, PB822, PN411, Faymex L-12(manufactured by Ajinomoto Fine-Techno Co., Inc.); DisperBYK (registeredtrademark) 101, 102, 106, 108, 111, 116, 130, 140, 142, 145, 161, 162,163, 164, 166, 167, 168, 170, 171, 174, 180, 182, 192, 193, 2000, 2001,2020, 2025, 2050, 2070, 2155, 2164, 220S, 300, 306, 320, 322, 325, 330,340, 350, 377, 378, 380N, 410, 425, 430 (manufactured by BYK Japan KK);Disparlon (registered trademark) 1751N, 1831, 1850, 1860, 1934, DA-400N,DA-703-50, DA-725, DA-705, DA-7301, DN-900, NS-5210, NVI-8514L(manufactured by Kusumoto Chemicals, Ltd.); Arufon (registeredtrademark) UC-3000, UF-5022, UG-4010, UG-4035, and UG-4070 (manufacturedby Toagosei Co., Ltd.); and the like may be listed.

The method of dispersing electromagnetic wave absorbing particles in aliquid medium is not limited in particular, as long as that the methodcan disperse the electromagnetic wave absorbing particles in the liquidmedium. At this time, it is favorable to carry out the dispersion sothat the mean particle diameter of the electromagnetic wave absorbingparticles becomes less than or equal to 100 nm, and it is more favorablethat the mean particle diameter becomes greater than or equal to 0.1 mand less than or equal to 100 nm.

As the method of dispersing electromagnetic wave absorbing particles ina liquid medium, methods of dispersion using devices such as a beadmill, a ball mill, a sand mill, a paint shaker, an ultrasonichomogenizer, and the like may be listed. Among these, it is favorable tocrush and disperse electromagnetic wave absorbing particles with amedium-stirring mill such as a bead mill, a ball mill, a sand mill, anda paint shaker that use media (beads, balls, Ottawa sand), from theviewpoint of shorting the time required to obtain the desired meanparticle diameter. By crushing and dispersing electromagnetic waveabsorbing particles with a medium-stirring mill, the electromagneticwave absorbing particles are dispersed in the liquid medium, and at thesame time, becomes finer particles caused by collision between theelectromagnetic wave absorbing particles and collision between themedium and the electromagnetic wave absorbing particles, and thereby,the electromagnetic wave absorbing particles can be made finer to bedispersed (i.e., crushed and dispersed).

It is favorable that the mean particle diameter of electromagnetic waveabsorbing particles is greater than or equal to 0.1 nm and less than orequal to 100 nm as described above. This is because a smaller meanparticle diameter decreases scattering of light in the visible lightregion of wavelengths from 400 nm to 780 nm due to geometric scatteringor Mie scattering; and as a result, it is possible to avoid the loss ofclear transparency, which would be caused by an electromagnetic waveabsorbing particle dispersoid like a cloudy glass, in whichelectromagnetic wave absorbing particles are dispersed in a resin. Inother words, when the mean particle diameter becomes less than or equalto 200 nm, the geometric scattering or Mie scattering described abovedecreases, and the scattering can be considered in a Rayleigh scatteringregion. In a Rayleigh scattering region, the scattered light isproportional to the sixth power of the dispersed particle diameter;therefore, as the dispersion particle diameter decreases, the scatteringdecreases and the transparency improves. Further, when the mean particlediameter becomes less than or equal to 100 nm, the scattered lightbecomes extremely insignificant, which is favorable.

Meanwhile, as for the dispersion state of electromagnetic wave absorbingparticles in an electromagnetic wave absorbing particle dispersoid inwhich the electromagnetic wave absorbing particles are dispersed in aresin and the like, which is obtained by using an electromagnetic waveabsorbing particle dispersion liquid according to the presentembodiment, as long as the dispersion liquid is added to the resin andthe like by a publicly-known method, cohesion does not occur to begreater than the mean particle diameter of the electromagnetic waveabsorbing particles in the dispersion liquid.

Also, as long as the mean particle diameter of electromagnetic waveabsorbing particles is greater than or equal to 0.1 nm and less than orequal to 100 nm, it is possible to prevent the electromagnetic waveabsorbing particle dispersoid to be manufactured and its molded products(plates, sheets, etc.) from becoming grayish products havingmonotonically decreased transmittance.

Although the content of electromagnetic wave absorbing particles in anelectromagnetic wave absorbing particle dispersion liquid according tothe present embodiment is not limited in particular, it is favorablethat the content is, for example, greater than or equal to 0.01 mass %and less than or equal to 80 mass %. This is because a sufficient solartransmittance can be achieved by setting the electromagnetic waveabsorbing particle content to be greater than or equal to 0.01 mass %.This is also because it is possible to disperse the particles uniformlyin the dispersion medium by setting to be less than or equal to 80 mass%.

Also, electromagnetic wave absorbing particles of the present embodimentcan also be used to make an electromagnetic wave absorbing particledispersoid.

An electromagnetic wave absorbing particle dispersoid according to thepresent embodiment can have a form in which the electromagnetic waveabsorbing particles are dispersed in a solid medium such as a resin.

The shape of an electromagnetic wave absorbing particle dispersoid isnot limited in particular, and may have any shape, for example, may havea film shape. In other words, an electromagnetic wave absorbing particledispersoid according to the present embodiment can be a film in which,for example, electromagnetic wave absorbing particles are dispersed in asolid medium such as a resin.

An electromagnetic wave absorbing particle dispersoid according to thepresent embodiment can be obtained by adding and melting a resin into anelectromagnetic wave absorbing particle dispersion liquid according tothe present embodiment, and pouring the obtained resin-addedelectromagnetic wave absorbing particle dispersion liquid into a mold ofany shape; or applying it to a base material that transmits visiblelight such as a glass plate, and then, solidifying the resin-addedelectromagnetic wave absorbing particle dispersion liquid. Also, anelectromagnetic wave absorbing particle dispersoid can also be obtainedby kneading an electromagnetic wave absorbing particle dispersion liquidaccording to the present embodiment into a resin.

The resin is not limited in particular, and, for example, a UV curableresin, a thermosetting resin, an electron-beam curable resin, aroom-temperature curable resin, a thermoplastic resin, or the like canbe selected in accordance with the purpose. Specifically, a polyethyleneresin, polyvinyl chloride resin, polyvinylidene chloride resin,polyvinyl alcohol resin, polystyrene resin, polypropylene resin,ethylene vinyl acetate copolymer, polyester resin, polyethyleneterephthalate resin, fluororesin, polycarbonate resin, acrylic resin,polyvinyl butyral resin, or ionomer resin may be listed. These resinsmay be used singly or in combination.

EXAMPLES

In the following, the present invention will be described specificallywith reference to examples. However, the present invention is notlimited to the following examples.

Evaluation methods in the following examples will be described.

(Chemical Analysis)

Chemical analysis of obtained electromagnetic wave absorbing particleswas carried out by atomic absorption spectroscopy (AAS) for Cs and ICPoptical emission spectrometry (ICP-OES) for W. Also, for O, a lightelement analyzer (model: ON-836, manufactured by LECO) was used to melta sample in He gas, and CO gas reacted with carbon in a crucible wasanalyzed by a method of determining the amount by IR absorptionspectroscopy. The chemical analysis was carried out three times for eachcomponent to calculate the standard deviation.

(X-Ray Diffraction Measurement)

X-ray diffraction measurement was carried out by powder XRD measurementusing Cu—K-α rays with a device called X′Pert-PRO/MPD manufactured bySpectris Co., Ltd.

The measurement was carried out for a range of angle 2θ greater than orequal to 10° and less than or equal to 100, with a scan speed at 0.095°(2θ)/sec, and measurement points being 5555 points/90°. The diffractionangle was calibrated with an Si standard sample (NIST640e) to carry outa Rietveld analysis assuming the space group P6₃/mcm or P6₃ cm, so as todetermine the lattice constants and atomic positions of a crystal phase.

Also, in Examples 1 to 10, the interatomic distances were calculatedbased on the obtained atomic positions of the crystal phase; among theW-O distances in a WO₆ octahedron, the ratio of the maximum value to theminimum value of distances from [an O atom present in the c-axisdirection when viewed from the W atom] to [the W atom at the center](the maximum value/the minimum value) was determined.

(Visible Light Transmittance, Solar Transmittance, and Near-InfraredTransmittance at the Wavelength of 850 nm)

The visible light transmittance, solar transmittance, and near-infraredtransmittance at the wavelength of 850 nm of electromagnetic waveabsorbing particles were measured according to ISO 9050 and JIS R 3106.Specifically, the transmittance was measured by using aspectrophotometer U-4100 manufactured by Hitachi, Ltd. and calculated bymultiplying by a coefficient corresponding to the spectrum of solarlight. The transmittance was measured at 5 nm intervals in a range ofwavelengths greater than or equal to 300 nm and less than or equal to2100 nm. Measurement steps will be described in detail later in Example1.

(Mean Particle Diameter)

The mean particle diameter was determined from a particle sizedistribution measured by using a transmission electron microscope(TECNAI G2 F20 manufactured by FEI Company).

Specifically, the particle diameter was measured for 300 particles froman observed image of the transmission electron microscope, and theparticle diameter at the cumulative value of 50% in the particle sizedistribution was determined as the mean particle diameter.

Example 1

As the raw materials, an aqueous solution of cesium carbonate (Cs₂CO₃)and tungstic acid (H₂WO₄) were weighed, mixed, and kneaded to prepare araw material mixture to be a ratio of Cs/W=0.33.

Then, the raw material mixture was dried in the atmosphere at 100° C.for 12 hours.

Then, 10 g of the dried raw material mixture (precursor) was spread flatand thinly over a carbon boat, heated and held (heating step) at 550° C.for 2 hours under a gas stream of 1 vol % H₂ gas with N₂ gas as acarrier (hereafter, also referred to as “1 vol % H₂—N₂”).

Next, after held at 550° C. for 1 hour under a 100 vol % N₂ gas stream,the temperature was raised to 800° C. and held at 800° C. for 1 hour tobe homogenized (homogenization step).

After the homogenization step, a blue powder of electromagnetic waveabsorbing particles was obtained.

The obtained blue powder was chemically analyzed independently forelements Cs, W, and O. As a result, values of 15.62±0.15 mass % for Cs,66.54±0.20 mass % for W, and 14.72±0.26 mass % for O were obtained. Thevalues correspond to 97.4% of the prepared amount for Cs, 99.9% of theprepared amount for W, and 84.7% of the prepared amount of O. These werenormalized to 100%, converted to at %, and the atomic number ratio wasobtained with W=1, to determine the composition of the obtained compoundas CS_(0.325)WO_(2.542).

In the general formula M_(x)WO_(3-y) of tungsten bronze described above,x=0.325, y=0.458, and 0 was found to be deficient by 15.3%.

As a result of XRD measurement with respect to the obtained blue powder,the powder matched JCPDS card No. 81-1224 and was identified asCs_(0.3)WO₃.

In order to further obtain detailed information on the structure, theRietveld analysis was carried out with the XRD patterns based on thespace group P6₃/mcm. Here, the site occupancy rate was 100% for W, 97%for Cs, and 50% as maximum for O(1), and the Wickoff coordinates for theelements were set as 6g site (x, 0, 0.25) for W, 12k site (x, 0, z); z=0for O(1), 12j site (x, y, 0.25) for O(2), and 2b site (0, 0, 0) for Cs.The isotropic temperature factor B (×10⁴ pm²) was assumed to be 0.2047for W, 0.07 for O(1), 0.10 for O(2), and 1.035 for Cs. At this time, asa reliability factor Rf defined by the following formula, a value ofRf=0.021 was obtained, which is the smallest value of cesium tungstenbronze compared with values in the past literature.

$\begin{matrix}{R_{f} = \frac{\sum\limits_{k}\left| {\sqrt{I_{k}\left( {obs} \right)} - \sqrt{I_{k}\left( {calc} \right)}} \right|}{\sum_{k}\sqrt{I_{k}\left( {obs} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Therefore, the analysis results in the present example are considered tobe the most reliable analysis to date. By this analysis, the latticeconstants were obtained as a=7.40686(4)(Å) and c=7.61015(6) (Å) (thestandard deviation of the last digit is in parentheses).

These values of lattice constants are positioned very close to thestraight line defined by the above-described expression c=−3.436a+33.062for the lattice constants a and c in the case of the element M being Cs.It was also confirmed that these were positioned in the vicinity of thepoint 216 in FIG. 2.

From the calculated lattice constants, the volume of a unit cell oftungsten bronze contained in electromagnetic wave absorbing particlesproduced in the present example was determined as 361.6 Å³; therefore,based on the molar ratio of oxygen obtained from the chemical analysis,the oxygen vacancy concentration N_(V) was found to be N_(V)=7.60×10²¹cm⁻³.

Then, the W-O distance in the WO₆ octahedron was determined from thelattice constants and atomic coordinates of the crystal phase obtainedby the Rietveld analysis. As a result, there are two distances from [anO atom present in the c-axis direction when viewed from the W atom] to[the W atom at the center], which were 1.903 Å and 1.920 Å,respectively, and the ratio of the maximum value to the minimum valuewas 1.01.

Next, 20 mass % of the powder, 10 mass % of an acryl-based polymericdispersant having an amine-containing group as a functional group, and70 mass % of methyl isobutyl ketone (MIBK) as a liquid medium wereweighed (hereafter, referred to as the “dispersant a”). These wereloaded into a paint shaker containing 0.3 mm φZrO₂ beads and crushed anddispersed for 10 hours to obtain an electromagnetic wave absorbingparticle dispersion liquid (hereafter, referred to as the “dispersionliquid A”). Here, the mean particle diameter of electromagnetic waveabsorbing particles in the dispersion liquid A was measured, and was22.1 nm.

The dispersion liquid A was diluted appropriately with MIBK and placedin a 10-mm-thick rectangular container to measure the spectraltransmittance. From the transmittance profile as measured by adjustingthe dilution rate so as to make the maximum transmittance become 95.5%(all maximum values were taken at around 500 nm), the visible lighttransmittance (VLT), the solar transmittance (ST), and the near-infraredtransmittance at the wavelength of 850 nm (T₈₅₀) were calculated orread. T₈₅₀ is a representative transmittance in near-infraredwavelengths, having a high sensor sensitivity. VLT=94.33%, ST=74.89%,and T₈₅₀=63.59% were obtained. It can be seen that while the visibletransmittance is very high, the solar transmittance is kept low, andhence, the near-infrared shielding effect is very high. Also, in thissample, as described above, the transmittance of 63.59% at the sensorwavelength of 850 nm was obtained.

The evaluation results are summarized in Table 1 and Table 2.

Examples 2 to 7

Powders of electromagnetic wave absorbing particles were produced insubstantially the same way as in Example 1 except that the hold time at550° C. in the heating step was changed to times shown in Table 1, andnext, in substantially the same way as in Example 1, the chemicalanalysis, measurement by XRD patterns, and Rietveld analysis using theXRD patterns were carried out for these powders.

Furthermore, electromagnetic wave absorbing particle dispersion liquidswere prepared in substantially the same way as in Example 1 except thatthe electromagnetic wave absorbing particles obtained in the respectiveExamples were used, to evaluate the mean particle diameter andspectrophotometric properties. These results are summarized in Tables 1and 2.

As shown in Tables 1 and 2, in Examples 2 to 7, the hold times in theheating step at 550° C. were set increasingly as 45 minutes, 50 minutes,60 minutes, 75 minutes, 3 hours, and 6 hours, respectively. Generally,in this order, the lattice constant decreases in the a-axis andincreases in the c-axis. In other words, in the case of the straightline illustrated in FIG. 2, it moved toward the upper left side. This isa change in a direction in which the Jahn-Teller distortion is graduallyalleviated. In addition, in the case of the hold time being 2 hours orlonger, the position was almost unchanged, indicating that thealleviation has reached an equilibrium state. The oxygen vacancyconcentration N_(V) also increased gradually up to the hold time of 75minutes, but remained around the same value for the hold time of 2 hoursor longer.

Next, as a result of the spectral characteristic evaluation of thedispersion liquids, as shown in Table 2, a tendency was observed thatwhile the oxygen vacancy concentration increases, the ST/VLT ratiodecreases. This indicates that the shielding effect of near-infraredlight relative to the visible light transmittance becomes stronger whilethe oxygen vacancy concentration increased.

At the same time, while the oxygen vacancy concentration increases, T₈₅₀decreases gradually and the sensor transmittance decreases gradually;however, T₈₅₀ decreases by a smaller extent than ST/VLT. Therefore, forexample, the amount of electromagnetic wave absorbing particles requiredto achieve the target solar transmittance can be decreased, and thesensor transmittance, namely, the near-infrared transmittance at thewavelength of 850 nm can be made higher.

Examples 8 to 10

When preparing a raw material mixture, in Examples 8 to 10, the rawmaterials were weighed, mixed, and kneaded so that Cs/W=0.30, 0.25, and0.20, respectively. Also, in the heating step, heating was carried outunder a stream of 0.7 vol % H₂ gas with N₂ gas as the carrier. Exceptfor the above points, electromagnetic wave absorbing particles wereprepared and evaluated in substantially the same way as in Example 4.

Furthermore, electromagnetic wave absorbing particle dispersion liquidswere prepared in substantially the same way as in Example 1 except thatthe electromagnetic wave absorbing particles obtained in the respectiveExamples were used, to evaluate the mean particle diameter andspectrophotometric properties. These results are summarized in Tables 1and 2.

According to the results shown in Tables 1 and 2, it can be seen thatthe lattice constants are changed such that while the value of Cs/W,namely, x decreases, the lattice constant in the a-axis directionincreases, and the lattice constant in the c-axis direction decreases.The extent of this change was far greater than in the case of changingthe amount y of oxygen deficiency in Examples 1 to 7. More specifically,[the amount of change in the lattice constants from Example 10 (x=0.196)to Example 3 (x=0.327)] was approximately 4.6 times greater than [theamount of change in the lattice constants from Example 2 (y=0.260) toExample 1 (y=0.458)]. In other words, it was confirmed that the addedamount x of the element(s) M has a large effect on the change in thelattice constants.

In Example 10, although the amount of x as Cs/W was measured to be0.196, a small amount of diffraction rays of WO₃, WO_(2.90), andWO_(2.93) as different phases were mixed in the powder XRD diffractionpatterns at this time.

Also, as for the electromagnetic wave absorbing particle dispersionliquid, as shown in Tables 1 and 2, a tendency was observed that whilethe value of Cs/W, namely, x, increases, the ST/VLT ratio decreases.This indicates that the shielding effect of near-infrared light relativeto the visible light transmittance becomes stronger while the value ofCs/W increases.

At the same time, it was confirmed that while Cs/W increases, T₈₅₀decreases gradually and the sensor transmittance decreased gradually.This is because the increase in Cs/W increases free electrons, increasesthe surface plasmon absorption, and the increase in the surface plasmonabsorption reaches up to the wavelength of 850 nm.

However, it was confirmed that the sensor transmittance was 58.94% evenin Example 8 as the lowest, which is not problematic in practice.

Example 11

A total of 30 g of anhydrous tungsten trioxide (WO₃), cesium tungstate(Cs₂WO₄), and a (metal) powder of W simple substance were weighed andmixed so that the atomic ratios became Cs/W=0.33 and O/W=3.00.

At the time of weighing, Cs₂WO₄ took in the atmospheric moisture andstained the drug packaging paper, but was weighed as quickly as possibleto be put into a 10-mm-diameter quartz tube and vacuum sealed, and then,heated and held at 750° C. for 3 days (heating and reduction step).

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4058 Å and c=7.6102 Å; values close to thoseof the electromagnetic wave absorbing particles in Example 1 wereobtained. In other words, electromagnetic wave absorbing particleshaving an amount of oxygen deficiency close to the maximum wereobtained.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the transmittance of the electromagnetic wave absorbingparticle dispersion liquid was 94.63%, the solar radiation was 75.27%,and the near-infrared transmittance at the wavelength of 850 nm was63.55%; values close to those in Example 1 were obtained.

Example 12

A total of 30 g of anhydrous tungsten trioxide (WO₃), cesium tungstate(Cs₂WO₄), and a (metal) powder of W simple substance were weighed andmixed so that the atomic ratios became Cs/W=0.33 and O/W=2.95. At thetime of weighing, Cs₂WO₄ took in the atmospheric moisture and stainedthe drug packaging paper, but was weighed and mixed as quickly aspossible, spread thinly over a carbon boat, put into a vacuumcalcination furnace, and then, heated and held at 750° C. for 3 days(heating and reduction step).

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4066 Å and c=7.6113 Å; values close to thoseof the electromagnetic wave absorbing particles in Example 1 wereobtained. In other words, electromagnetic wave absorbing particleshaving an amount of oxygen deficiency close to the maximum wereobtained.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the transmittance of the electromagnetic wave absorbingparticle dispersion liquid was 94.52%, the solar radiation was 74.91%,and the near-infrared transmittance at the wavelength of 850 nm was63.49%; values close to those in Example 1 were obtained.

Example 13

A total of 30 g of anhydrous tungsten trioxide (WO₃), cesium tungstate(Cs₂WO₄), and a (metal) powder of W simple substance were weighed andmixed so that the atomic ratios became Cs/W=0.33 and O/W=3.00.

Note that the raw material Cs₂WO₄ was held in advance in the calcinationfurnace at 200° C. for 1 hour to remove the moisture, and was weighed asquickly as possible.

Then, the raw material mixture was spread flat and thinly over analumina boat, placed in a vacuum calcination furnace, and heated andheld at 750° C. for 3 days (heating and reduction step).

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4078 Å and c=7.6086 Å; values close to thoseof the electromagnetic wave absorbing particles in Example 5 wereobtained. In other words, compared to Example 1 and the like, withrespect to the extent of reduction, electromagnetic wave absorbingparticles in a slightly alleviated state of reduction were obtained.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible light transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 94.78% and the solar radiationtransmittance was 76.14%; values close to those in Example 5 wereobtained. However, although the near-infrared shielding became strongerthan in Example 5 as ST/VLT=0.803, the near-infrared transmittance atthe wavelength of 850 nm was 67.84%; it was confirmed that there wasalmost no difference from Example 5. In other words, if compared for thesame near-infrared shielding effect, the sensor transmittance isslightly improved compared to Example 5.

Example 14

A total of 30 g of anhydrous tungsten trioxide (WO₃), cesium tungstate(Cs₂WO₄), and a (metal) powder of W simple substance were weighed andmixed so that the atomic ratios became Cs/W=0.33 and O/W=3.00.

Note that the raw material Cs₂WO₄ was held in advance in a calcinationfurnace at 200° C. for 1 hour to remove the moisture, carefullyintroduced into a dried glove box to be weighed and mixed with the otherraw materials.

Then, the raw material mixture was spread flat and thinly over analumina boat, placed in a vacuum calcination furnace, and heated andheld at 750° C. for 2 days (heating and reduction step).

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4106 Å and c=7.5978 Å, which are values closeto the point 212 among the points illustrated in FIG. 2. In other words,a sample having a low amount y of oxygen deficiency was obtained.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 95.14% and the solartransmittance was 77.90%. Also, ST/VLT=0.819, slightly larger than inExample 3. However, the near-infrared transmittance at the wavelength of850 nm was 81.46%; it was confirmed to be even higher than in Example 3.Therefore, it was confirmed that electromagnetic wave absorbingparticles were obtained that are particularly effective when it isdesired to increase the sensor transmission.

Example 15

A total of 30 g of anhydrous tungsten trioxide (WO₃), cesium tungstate(Cs₂WO₄), and a (metal) powder of W simple substance were weighed andmixed so that the atomic ratios became Cs/W=0.33 and O/W=3.00.

Note that the raw material Cs₂WO₄ was held in advance in a calcinationfurnace at 200° C. for 1 hour to remove the moisture, carefullyintroduced into a dried glove box to be weighed and mixed with the otherraw materials.

Then, the raw material mixture was spread flat and thinly over analumina boat, placed in a calcination furnace, and heated and held at750° C. for 2 days under a gas stream of 100% nitrogen gas (heating andreduction step).

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4091 Å and c=7.5998 Å, which are values closeto the point 213 among the points illustrated in FIG. 2. In other words,a sample with a relatively small amount y of oxygen deficiency wasobtained.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 95.25% and the solartransmittance was 80.78%. Also, the near-infrared transmittance at thewavelength of 850 nm was 76.49%; it was confirmed to be even higher thanin Example 2.

Example 16

Blue electromagnetic wave absorbing particles Cs_(0.324)WO_(2.542) in afully-reduced state produced in Example 1 were placed in an aluminaboat, and annealed at 500° C. for 30 minutes in the atmosphere as anoxidizing gas atmosphere (annealing step).

The obtained electromagnetic wave absorbing particles after theannealing step were fairly whitish.

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4135 Å and c=7.5886 Å; it was confirmed thatthe line illustrated in FIG. 2 was moved toward the lower right side.Note that annealing was carried out in the atmosphere; therefore, it canbe considered that oxygen became excessive and the lattice constantschanged greatly.

Furthermore, the electromagnetic wave absorbing particles after theannealing step were placed in a tubular furnace at 500° C., throughwhich 1 vol % H₂—N₂ gas was flowed, held for 30 minutes, and then, takenout (the annealing step).

Then, the obtained electromagnetic wave absorbing particles were foundto be slightly bluish in color.

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.4110 Å and c=7.5962 Å; it was confirmed to bepositioned virtually between the points 211 and 212 of the lineillustrated in FIG. 2. In other words, it was confirmed that the amountof oxygen deficiency became smaller.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 95.21% and the solartransmittance was 83.40%. Also, ST/VLT=0.876, which is substantially thesame as in Example 15. However, the near-infrared transmittance at thewavelength of 850 nm was 80.38%; it was confirmed to be even higher thanin Example 15. Therefore, it was confirmed that electromagnetic waveabsorbing particles were obtained that are particularly effective whenit is desired to increase the sensor transmission.

Example 17

As the raw materials, an aqueous solution of rubidium carbonate (Rb₂CO₃)and tungstic acid (H₂WO₄) were weighed, mixed, and kneaded to prepare araw material mixture to be a ratio of Rb/W=0.33.

Then, the raw material mixture was dried in the atmosphere at 100° C.for 12 hours.

Then, 10 grams of the dried raw material mixture (precursor) was spreadflat and thinly over a carbon boat, heated and held and at 550° C. for 2hours under a gas stream of 1 vol % H₂ gas with N₂ gas as a carrier(heating step).

Next, after held at 550° C. for 1 hour under a 100 vol % N₂ gas stream,the temperature was raised to 800° C. and held at 800° C. for 1 hour tobe homogenized (homogenization step).

After the homogenization step, a blue powder was obtained.

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.3854 Å and c=7.5677 Å; it was confirmed thatit was positioned virtually on the dashed line 11 illustrated in FIG. 1.

Also, the chemical analysis was carried out in substantially the sameway as in Example 1, to determine the composition and oxygen vacancyconcentration, and x as the value of Rb/W was 0.326. Also, the oxygenvacancy concentration N_(V) was determined to be 4.57×10²¹ cm⁻³. It wasconfirmed that a lower oxygen vacancy concentration than in the case ofCs_(x)WO_(3-y) was obtained even in a similar process.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 94.53% and the solartransmittance was 76.23%. Also, as ST/VLT=0.806, a good near-infraredshielding effect close to Example 1 was obtained. Also, thenear-infrared transmittance at the wavelength of 850 nm was 76.27%; itwas confirmed that a high value was obtained.

Example 18

As the raw materials, an aqueous solution of potassium carbonate (K₂CO₃)and tungstic acid (H₂WO₄) were weighed, mixed, and kneaded to prepare araw material mixture to be a ratio of K/W=0.25.

Then, the raw material mixture was dried in the atmosphere at 100° C.for 12 hours.

Then, 10 grams of the dried raw material mixture (precursor) was spreadflat and thinly over a carbon boat, heated and held and at 550° C. for 1hour under a gas stream of 5 vol % H₂ gas with N₂ gas as a carrier(heating step).

Next, after held at 550° C. for 1 hour under a 100 vol % N₂ gas stream,the temperature was raised to 700° C. and held at 700° C. for 1 hour tobe homogenized (homogenization step).

After the homogenization step, a blue powder was obtained.

The XRD measurement was carried out with respect to the obtainedelectromagnetic wave absorbing particles, and as a result of determiningthe lattice constants, a=7.3818 Å and c=7.5413 Å; it was confirmed thatit was positioned virtually on the dashed line 12 illustrated in FIG. 1.

Also, the chemical analysis was carried out in substantially the sameway as in Example 1, to determine the composition and oxygen vacancyconcentration, and x as the value of Rb/W was 0.248. Also, the oxygenvacancy concentration N_(V) was determined to be 4.13×10²¹ cm⁻³. It wasconfirmed that a lower oxygen vacancy concentration than in the case ofCs_(x)WO_(3-y) was obtained even in a similar process.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 94.98% and the solartransmittance was 83.16%. Also, although ST/VLT=0.876 was relativelyhigh, a near-infrared shielding effect close to Example 16 was obtained.Also, the near-infrared transmittance at the wavelength of 850 nm was86.21%; it was confirmed that a high value was obtained.

Example 19

As the raw materials, an aqueous solution of cesium carbonate (Cs₂CO₃),an aqueous solution of rubidium carbonate (Rb₂CO₃), and tungstic acid(H₂WO₄) were weighed, mixed, and kneaded to prepare a raw materialmixture to be ratios of Cs/W=0.13 and Rb/W=0.20.

Then, the raw material mixture was dried in the atmosphere at 100° C.for 12 hours.

Then, 10 grams of the dried raw material mixture (precursor) was spreadflat and thinly over a carbon boat, heated and held and at 550° C. for 1hour under a gas stream of 5 vol % H₂ gas with N₂ gas as a carrier(heating step).

Next, after held at 550° C. for 1 hour under a 100 vol % N₂ gas stream,the temperature was raised to 700° C. and held at 700° C. for 1 hour tobe homogenized (homogenization step).

After the homogenization step, a blue powder was obtained.

The XRD measurement was carried out for the obtained electromagneticwave absorbing particles, diffraction rays that indicate a singlehexagonal crystal were obtained, and it was confirmed that obtained wasa solid solution of Cs and Rb. The lattice constants were determined tobe a=7.3960 Å and c=7.5821 Å.

Also, the chemical analysis was carried out in substantially the sameway as in Example 1, to determine the composition and oxygen vacancyconcentration, and x as the value of M/W was 0.325. Also, the oxygenvacancy concentration N_(V) was determined to be 5.81×10²¹ cm⁻³.

Furthermore, an electromagnetic wave absorbing particle dispersionliquid was prepared in substantially the same way as in Example 1 exceptthat electromagnetic wave absorbing particles obtained in the presentexample were used, to evaluate the mean particle size andspectrophotometric characteristics. These results are summarized inTables 1 and 2.

Note that the visible transmittance of the electromagnetic waveabsorbing particle dispersion liquid was 94.65% and the solartransmittance was 75.81%. Also, as ST/VLT=0.801, a good near-infraredshielding effect close to Example 1 and Example 5 was obtained. Also,the near-infrared transmittance at the wavelength of 850 nm was 72.54%;it was confirmed that a value higher than in Example 1 and in Example 5was obtained.

TABLE 1 Characteristics of electromagnetic-wave-absorbing particles HoldValues of Oxygen W-O time x and y in Reliability Lattice vacancydistance Mean at general formula factor constants concentration Maximum/particle 550° C. M_(x)WO_(3-y) (Rf) a c (Nv) minimum diameter (min) x y— (Å) (Å) (10²¹ cm⁻³) — (nm) Example 1 120 0.325 0.458 0.021 7.40687.6101 7.60 1.01 22.1 Example 2 45 0.326 0.260 0.037 7.4088 7.6032 4.321.01 25.8 Example 3 50 0.327 0.306 0.034 7.4083 7.6038 5.09 1.01 23.7Example 4 60 0.322 0.369 0.025 7.4075 7.6057 6.13 1.01 21.7 Example 5 750.323 0.430 0.023 7.4078 7.6071 7.14 1.01 21.6 Example 6 180 0.324 0.4140.021 7.4068 7.6100 7.63 1.01 38.1 Example 7 360 0.291 0.333 0.0217.4064 7.6096 7.36 1.01 24.5 Example 8 60 0.295 0.269 0.035 7.40907.5969 4.48 1.02 24.3 Example 9 60 0.247 0.242 0.049 7.4155 7.5818 4.031.03 28.0 Example 10 60 0.196 0.321 0.119 7.4175 7.5719 5.34 1.03 33.6Example 11 0.322 0.456 0.032 7.4058 7.6102 7.59 34.6 Example 12 0.3240.458 0.028 7.4066 7.6113 7.62 38.3 Example 13 0.326 0.401 0.032 7.40787.6086 7.18 35.1 Example 14 0.324 0.070 0.026 7.4106 7.5978 1.41 36.7Example 15 0.321 0.185 0.024 7.4091 7.5998 2.96 33.6 Example 16 0.3230.034 0.027 7.4110 7.5962 1.17 36.2 Example 17 0.326 0.201 0.025 7.38547.5677 4.57 21.7 Example 18 0.248 0.083 0.041 7.3818 7.5413 4.13 29.4Example 19 0.325 0.367 0.037 7.3960 7.5821 5.81 23.9

TABLE 2 Thermoplastic resin used for electromagnetic-wave-absorbingsheet Optical characteristics of electromagnetic-wave-absorbing sheetand electromagnetic-wave-absorbing transparent base material Nearinfrared Visible light Solar transmittance at transmittancetransmittance wavelength 850 nm (VLT) (ST) (T₈₅₀) (%) (%) (%) ST/VLTExample 1 94.33 74.89 63.59 0.794 Example 2 95.37 79.57 75.36 0.834Example 3 94.93 77.27 71.27 0.814 Example 4 95.09 77.03 70.13 0.810Example 5 94.81 76.74 67.61 0.809 Example 6 94.70 74.80 64.22 0.790Example 7 93.78 73.15 60.75 0.780 Example 8 93.33 71.29 58.94 0.764Example 9 93.54 74.97 67.18 0.801 Example 10 93.10 75.14 71.29 0.807Example 11 94.63 75.27 63.55 0.795 Example 12 94.52 74.91 63.49 0.793Example 13 94.78 76.14 67.84 0.803 Example 14 95.14 77.90 81.46 0.819Example 15 95.25 80.78 76.49 0.848 Example 16 95.21 83.40 80.38 0.876Example 17 94.53 76.23 76.27 0.806 Example 18 94.98 83.16 86.21 0.876Example 19 94.65 75.81 72.54 0.801

As above, electromagnetic wave absorbing particles, manufacturingmethods of electromagnetic wave absorbing particles, and electromagneticwave absorbing particle dispersion liquid have been described withreference to the embodiments, examples, and the like. Note that thepresent invention is not limited to the embodiments, examples, and thelike described above. Various modifications and changes can be madewithin the scope of the gist of the present invention described in theclaims.

The present application is based on and claims priority to JapanesePatent Application No. 2017-154812 filed with the Japan Patent Office onAug. 9, 2017, and the entire contents of Japanese Patent Application No.2017-154812 are incorporated herein by reference.

1. Electromagnetic wave absorbing particles comprising: hexagonaltungsten bronze having oxygen deficiency, wherein the tungsten bronze isexpressed by a general formula: M_(x)WO_(3−y) (where one or moreelements M include at least one or more species selected from among K,Rb, and Cs, 0.15≤x≤0.33, and 0<y≤0.46), and wherein oxygen vacancyconcentration N_(V) in the electromagnetic wave absorbing particles isgreater than or equal to 4.3×10¹⁴ cm⁻³ and less than or equal to8.0×10²¹ cm⁻³.
 2. The electromagnetic wave absorbing particles asclaimed in claim 1, wherein the one or more elements M further includeone or more species selected from among Na, Tl, In, Li, Be, Mg, Ca, Sr,Ba, Al, and Ga as an additive element.
 3. The electromagnetic waveabsorbing particles as claimed in claim 1, wherein a_(M-HTB)(Å) andc_(M-HTB)(Å) as lattice constants a and c of the hexagonal tungstenbronze, in a coordinate space wherein a horizontal axis represents thelattice constant a(Å), a vertical axis represents the lattice constantc(Å), to represent a coordinate position by (the lattice constant a, thelattice constant c), have following relationships of formulas (1) and(2) with a point (a_(M), c_(M)) positioned in a quadrangular region ABCDconnecting points A(7.406, 7.614), B(7.372, 7.564), C(7.393, 7.504), andD(7.423, 7.554):a _(M-HTB) =a _(M)±0.0084  (1)c _(M-HTB) =c _(M)±0.0184  (2).
 4. The electromagnetic wave absorbingparticles as claimed in claim 1, wherein one or more elements M isconstituted with Cs, and wherein a_(M-HTB)(Å) and c_(M-HTB)(Å) as thelattice constants a and c of the hexagonal tungsten bronze, in acoordinate space wherein a horizontal axis represents a lattice constanta(Å), a vertical axis represents a lattice constant c(Å), to represent acoordinate position by (the lattice constant a, the lattice constant c),have following relationships of formulas (3) and (4) with a point(a_(Cs), c_(Cs)) positioned on a straight line ofc_(Cs)=−3.436a_(Cs)+33.062:a _(M-HTB) =a _(Cs)±0.0084  (3)c _(M-HTB) =c _(Cs)±0.0184  (4).
 5. The electromagnetic wave absorbingparticles as claimed in claim 4, wherein said c_(Cs) satisfies7.576≤c_(CS)≤7.610.
 6. The electromagnetic wave absorbing particles asclaimed in claim 1, wherein one or more elements M is constituted withRb, and wherein a_(M-HTB)(Å) and c_(M-HTB)(Å) as the lattice constants aand c of the hexagonal tungsten bronze, in a coordinate space wherein ahorizontal axis represents a lattice constant a(Å), a vertical axisrepresents a lattice constant c(Å), to represent a coordinate positionby (the lattice constant a, the lattice constant c), have followingrelationships of formulas (5) and (6) with a point (a_(Rb), c_(Rb))positioned on a straight line of c_(Rb)=−3.344a_(Rb)+32.265:a _(M-HTB) =a _(Rb)±0.0084  (5)c _(M-HTB) =c _(Rb)±0.0184  (6).
 7. The electromagnetic wave absorbingparticles as claimed in claim 6, wherein said c_(Rb) satisfies7.517≤c_(Rb)≤7.580.
 8. The electromagnetic wave absorbing particles asclaimed in claim 1, wherein one or more elements M is constituted withK, and wherein a_(M-HTB)(A) and c_(M-HTB)(Å) as the lattice constants aand c of the hexagonal tungsten bronze, in a coordinate space wherein ahorizontal axis represents a lattice constant a(Å), a vertical axisrepresents a lattice constant c(Å), to represent a coordinate positionby (the lattice constant a, the lattice constant c), have followingrelationships of formulas (7) and (8) with a point (a_(K), c_(K))positioned on a straight line of c_(K)=−2.9391a_(K)+29.227:a _(M-HTB) =a _(K)±0.0084  (7)c _(M-HTB) =c _(K)±0.0184  (8).
 9. The electromagnetic wave absorbingparticles as claimed in claim 8, wherein said c_(K) satisfies7.504≤c_(K)≤7.564.
 10. The electromagnetic wave absorbing particles asclaimed in claim 1, wherein in a WO₆ octahedron present in a crystal ofthe hexagonal tungsten bronze, a ratio of a maximum value to a minimumvalue among distances from [an O atom present in the c-axis directionwhen viewed from the W atom] to [the W atom at the center] is greaterthan or equal to 1.00 and less than or equal to 1.10.
 11. Theelectromagnetic wave absorbing particles as claimed in claim 1, whereina mean particle diameter is greater than or equal to 0.1 nm and lessthan or equal to 100 nm.
 12. The electromagnetic wave absorbingparticles as claimed in claim 1, wherein surfaces of the electromagneticwave absorbing particles are modified with a compound containing one ormore species of elements selected from among Si, Ti, Zr, and Al.
 13. Anelectromagnetic wave absorbing particle dispersion liquid, comprising:the electromagnetic wave absorbing particles as claimed in claim 1; anda liquid medium being one or more species selected from among water,organic solvent, oil and fat, a liquid resin, and a liquid plasticplasticizer, wherein the electromagnetic wave absorbing particles aredispersed in the liquid media.
 14. The electromagnetic wave absorbingparticle dispersion liquid as claimed in claim 13, wherein content ofthe electromagnetic wave absorbing particles is greater than or equal to0.01 mass % and less than or equal to 80 mass %.
 15. (canceled) 16.(canceled)
 17. (canceled)